Ultrasonic flowmeters are already known which comprise two acoustic probes disposed on a line inclined with respect to the axis of a conduit in which flows a fluid whose rate of flow is to be measured, these two probes functioning successively as transmitters then receivers. These probes are cylindrical in form and their active end, in intimate contact with the fluid, either penetrates directly in the flow of fluid, or is recessed with respect to the inner surface of the conduit.
In the first case, where the active end of the probes penetrates directly in the flow of fluid, the probes may be damaged by a solid body taken along by the fluid. Furthermore, the end of the probes may disturb the flow of the fluid and provoke a phenomenon of cavitation causing the appearance, at this end, of a gaseous cushion harmful to the acoustic transmission.
In the second case, where the active end of the probes is recessed with respect to the inner surface of the conduit, an air bubble may be trapped in front of the end of the probes, when the corresponding flowmeters are filled, which may prevent any acoustic transmission. Furthermore, still in this latter case, when the fluid is a cryogenic fluid, a possible temperature gradient on the wall of the conduit may provoke the vaporisation of the fluid and consequently further prevent the acoustic transmission.
Different solutions to these problems of flowmeters have already been proposed, particularly in French Pat. Nos. 1,245,961 and 1,604,378 and U.S. Pat. No. 3,906,791, said latter describing, column 7, line 65 to column 8, line 17, with reference to FIG. 11, an ultrasonic flowmeter of the type mentioned hereinabove, in which the flow is not disturbed and in which any temperature gradient on the screen constituting said means ensuring continuity of the flow of fluid is virtually eliminated since both sides of this screen bathes in the same fluid.
However, the flowmeter of U.S. Pat. No. 3,906,791 in fact comprises two recesses which, in addition, are constituted by bores formed in the thickness of the wall of the conduit. Now, the piercing of these bores is a costly operation, and the fact that the conduit is provided with these two separate recesses, and therefore with two screens associated therewith, does not ensure a total absence of temperature gradient.
It is therefore an object of the present invention to remedy the above-mentioned drawbacks.